A known optical fibre communications network is the passive optical network (PON) which is used to support voice and data traffic--so-called telephony over a passive optical network (TPON). Managing the movement of traffic through a TPON involves a number of technical considerations.
TPON systems currently are designed to carry a range of voice/data services. At the time these were specified, it was assumed that any expansion of such systems to include video would be a long way into the future for regulatory reasons. The systems were, therefore, specified to allow the use of a second wavelength for broadband services on the PON at a later date, without impinging on existing systems.
The present assignee has developed a bit transport system (BTS) for use in a TPON. In this bit transport, system, an optical line terminal (OLT) at an exchange transmits bit interleaved time division multiplex (TDM) frames downstream to all the receiving terminations on the network, known as optical network units (ONUs). The transmitted frames include both traffic data and control data. Each termination recognises, and responds to, appropriately-addressed portions of the data in the transmitted frames, and ignores the remainder of the frames.
In the upstream direction, each termination transmits data in a predetermined time slot, and the data from the different terminations are assembled at the OLT into time division multiple access (TDMA) frames of predetermined format.
One feature necessary to such a network is the provision of compensation for the differing delays associated with the different distances of the various terminations from the OLT. To this end, in the BTS system, each termination is arranged to transmit a ranging pulse timed to arrive in a predetermined portion of the upstream TDMA frame. The OLT is arranged to monitor the timing and phase of the arrival of the ranging pulse from each of the terminations, and to return a servo-control signal to each termination to adjust the launch power of that termination, and to retard or advance its transmissions as appropriate. This active fine ranging enables the BTS to ensure the stability of the upstream TDMA frame and, for example, to compensate for fluctuations in timing due to such effects as changes in the operating temperature of the network. However, this places severe demands on the design of the OLT, requiring the measurement of the timing of received signals to within a fraction of a clock cycle in real-time.
Additionally, the BTS must respond to commands from the next level in the network management hierarchy to allocate traffic circuits and to handle the addition/deletion of subscribers and the change/reallocation of numbers (known as "churn"). In practice, BTS controllers have no intrinsic knowledge of the type of traffic to be transmitted from an exchange (or its format), or the bandwidth which should be allocated to a particular network customer termination; it is up to the network management hierarchy to provide the BTS with all the data necessary to enable it to map the appropriate number of traffic bits to a specified circuit.
Currently, the BTS is designed to be substantially symmetrical. Bandwidth configured in the downstream direction is also available in the upstream direction.
In summary, the BTS is a transport system which allows the bandwidth from an OLT to be distributed flexibly between a number of remote customer ONUs sharing a common point-to-multipoint passive split optical network. In current TPON systems, four BTS master units at the OLT are connected, via a time switch, to tributary units (TUs) which enables any 64k timeslot from any 2048 Kbit/s TU to be mapped to any timeslot on a particular BTS master unit. The BTS slave in the ONU distributes the TPON bandwidth to service units (SUs) which deliver the individual 64k timeslots to the customer for any given service.
The concept of video-on-demand (VoD) has recently been proposed. When using VoD, a subscriber to the service would be able to call up a video transmission from a library of titles as and when the subscriber wanted to, and to manipulate the video information, for example by freezing on a particular frame and fast searching backwards and forwards.
Now that video compression techniques have been developed, requiring 2 Mbit/s or less per customer channel, the possibility of distributing video channels among customers is feasible, without the need for higher bandwidth transmission systems than are currently available.